This invention relates to additives for cement compositions. More particularly, this invention relates to corrosion inhibiting additives for Portland cements which combine a corrosion inhibitor with a set retarder and to the use of said additives in inhibiting the corrosion of metal in contact with the cement.
Calcium nitrite is well known as a corrosion inhibiting additive for Portland cements. U.S. Pat. No. 3,427,175, for example, relates to Portland cement compositions containing from about 0.1 to 10 percent calcium nitrite based on the dry weight of Portland cement and discloses that calcium nitrite inhibits corrosion of iron and steel reinforcing structures in the cement. While other nitrites, e.g., sodium nitrite, can be used to inhibit corrosion, calcium nitrite is preferred inasmuch as it provides effective corrosion inhibition without many of the disadvantages encountered with other nitrites, such as reduction in compressive strength or efflorescence on brick work.
When calcium nitrite is used in Portland cement compositions in the concentrations generally necessary to achieve substantial corrosion inhibition, e.g., greater than 0.5% by weight of calcium nitrite, based on the weight of dry cement, it normally also provides a substantial level of set acceleration. While this may be advantageous in certain applications, the set acceleration provided by a corrosion inhibiting amount of calcium nitrite can result in too rapid a setting of the cement composition. Particularly in warm weather applications, the set acceleration resulting from calcium nitrite addition can be an acute problem, resulting in extremely short setting times and, thus, severely foreshortening the time allowed for preparation, transport, working, etc., of the cement composition.
The severity of the set acceleration by corrosion inhibiting concentrations of calcium nitrite has heretofore been lessened by separately adding to the cement composition a set retarding agent which is capable of offsetting the set acceleration and extending the plastic life of the cement. However, this practice requires the cement producer or user to purchase, store, and dispense two additives, with added inconvenience, labor, and cost. Moreover, the need for a second additive is undesirable in that it increases the opportunity for error or miscalculation in dispensing the proper respective amounts of the additives.